This invention relates to an improved sucker rod formed from cold-drawn steel.
In the field of oil well pumping, sucker rods are rods which are joined together to extend down an oil well hole for the purpose of operating a pumping mechanism at the bottom of the well. Individual rods typically of 25-30 feet in length, and 1/2 inch to 11/4 inch in diameter, are connected end to end to form a relatively straight line or string within the oil well hole. Reciprocating motion from a pumping unit on the surface is transmitted via the string to a subsurface pump located at the bottom of the well hole, the pump serving to force the oil up through the same well hole to the surface. A string of sucker rods can be of substantial length, extending several miles long when installed.
In general, a sucker rod is made from a single piece of steel which has been hot upset forged to form the pin or box end shapes, or from two machined or cast end portions and a rod body member which are combined, typically through the use of a screw fit, to form a sucker rod.
The joining of sucker rods forming a string is ordinarily done in two ways, i.e., both ends of the sucker rods can be externally threaded to form matching pin ends, with an internally match-threaded coupling element joining two adjacent sucker rods in end to end relationship; or the sucker rod can be made with a single externally threaded pin end and a box formed at the other end which is internally match-threaded, the box being adapted to cooperate with the pin of an adjacent rod to directly join the ends of the sucker rods together.
A sucker rod string is in a constant state of tension, alternating between compression forces on the downward stroke of the surface pump and load forces on the upward stroke. Because of the cyclical stressing of the joints between adjacent sucker rods and of the sucker rod itself, weak points along the string are subject to metal fatigue failures. These tension stresses are further aggravated by bending stresses associated with fluid pound which result in cracking of the steel rod material, as well as deterioration of the rods from corrosive materials in the wells, and abrasion of the rods against the well casing or tubing. In general, however, the load force imposed on the string is responsible for the ultimate failure of a sucker rod. A failure of a rod or coupling causing a separation of the string results in costly down-time of the well, as well as the difficult and time consuming task of fishing out the dissociated portion of the string and replacing the component which has failed.
The threads formed on a sucker rod are locations of high stress concentration due to their shape. During the operation of the well, high tensional pump forces are also transmitted through the engaged threads. The coupling point between adjacent sucker rods in a string is thus a principal location of stress concentration, and the stress points are multiplied in sucker rods which are composed of multiple elements where the joining points also become a focal area of stress forces. Considerable effort has thus been expended to produce a sucker rod which minimizes the sensitivity of these threaded coupling and joint areas to metal fatigue from pumping stresses which can result in weakness or failure of the total assembly.
Steel for sucker rods has generally been made by either open hearth or electric induction furnace hot forging processes. The hot rolled bars formed in these processes are sheared to length at a rolling mill and machine straightened. In making a sucker rod from a single bar, both ends of the bar are separately heated to around 2400.degree. F., and upset forged to the pin or box shape required. For example, the upsetting process for a pin end would comprise forming a pin blank for the male end, gathering the metal into the pin shoulder and the wrench square shape, and forming the tapered bead or flare portion of the transition area between the sucker rod body and the upset end portion. This latter transition area is an area of high stress on the sucker rod. The sucker rod is then heat treated full length to improve tensile and yield strength, this treatment consisting of either normalizing, normalizing and tempering, or liquid quenching and tempering. The treated rods are then cleaned by shot or grit-blasting and the ends are machined and threaded.
Alternatively, it is known in the art to make a sucker rod from multiple elements of hot rolled steel. The hot rolled bars are sheared to length and machine straightened. Both ends of the bar are then externally threaded. Pin and box portions for the bar ends are either cast or machined to the proper shape, and internally threaded to match the ends of the bar. The appropriate end elements are then screwed on to the bar and torque fitted. The joint thus formed in the transition area between the sucker rod body and the end element is a weak point caused by both stress concentration and lack of fatigue resistance on this area.